The present invention relates to a process and to a plant for producing argon by cryogenic distillation.
Usually, an argon-enriched stream is withdrawn from the low-pressure column of a double air-separation column and sent to the bottom of a first column so as to decrease as much as necessary the amount of argon contained in the oxygen produced.
In the conventional process, the argon produced by this first column is withdrawn from the top and it essentially contains the nitrogen introduced in the feed.
It is therefore common practice, as may be seen in EP-A-0 669 509 and EP-A-0669508, to send it into a second column called a denitrogenating column comprising a stripping section intended to remove the nitrogen from the argon produced at the bottom, a rectifying section intended to remove the argon from the waste nitrogen withdrawn from the top, a bottom reboiler and an overhead condenser.
U.S. Pat. No. 5,133,790 describes a system in which enough distillation trays are fitted in the low-pressure column, above the point where feed is withdrawn from the first column, in order to lower the nitrogen content in proportions such that the argon produced has a xe2x80x9ccommercialxe2x80x9d argon content, generally 1 ppm. Thus, the denitrogenating column may be omitted. It is therefore useful to have several distillation trays above the point of withdrawal of the argon-rich fluid from the first column and to provide a purge at the top of the first column so as to remove a portion of the nitrogen introduced. Under these conditions, the nitrogen content of the argon produced may be approximately three times less than that of the column feed.
The drawback of this process is that it is essential to be able to control the operation of the low-pressure column so that the nitrogen content at the point of withdrawal of the feed intended for the first column never exceeds the permissible amount, unless the column has enough trays to have at the nominal point a sufficient margin with respect to this limit.
U.S. Pat. Nos. 4,977,746 and 4,824,453 disclose a process for producing ultrapure oxygen and argon in which a liquid withdrawn at an intermediate level of the argon production column feeds the top of a stripping column at the bottom of which the ultrapure oxygen forms.
It is an object of the invention to provide a process for separating an argon-enriched gas by cryogenic distillation, comprising the following steps:
the step of sending an argon-enriched gas stream to a first column;
the step of separating the gas by distillation in the first column into an oxygen-enriched liquid and an argon-enriched gas;
the step of at least partially condensing the argon-rich gas in an overhead condenser of the first column in order to form reflux;
the step of removing another argon-rich stream containing at most 1000 ppm oxygen and 1000 ppm nitrogen from the first column and of sending it to the top of a second column;
the step of warming the bottom of the second column by means of a warming gas; and
the step of withdrawing, as final product, an argon-rich fluid from the lower portion of the second column.
The stream sent to the top of the second column may be a gas or a liquid.
Preferably, the warming gas is a fraction of the argon-enriched gas which feeds the first column.
In certain cases, the first column is in two sections, one section being fed with the argon-enriched gas and the other having the overhead condenser.
Normally, the first column is fed from the low-pressure column of a double column.
Preferably, an overhead gas of the second column is sent either to the top of the first column or to the overhead condenser of the first column.
It is another object of the invention to provide a plant for separating an argon-enriched gas by cryogenic distillation, comprising:
a first column having an overhead condenser;
a second column having a bottom reboiler;
means for sending an argon-enriched gas to a first level of the first column;
means for withdrawing an argon-rich fluid at a second level of the first column;
means for sending the argon-rich fluid to the top of the second column;
means for sending a warming gas to the bottom reboiler;
means for withdrawing an argon-rich final product from the bottom of the second column,
characterized in that, between the first and second levels, there are at least 100 theoretical trays.
The argon-rich fluid sent to the top of the second column may be a gas or a liquid.
Preferably, the plant comprises means for sending a portion of the argon-enriched gas to the bottom reboiler as warming gas.
Optionally, the first column is constructed in two sections. Preferably, the plant comprises means for sending an overhead gas from the second column either to the top of the first column or to the overhead condenser of the first column.